It doesn't matter how they give mice alzheimers, or any other condition. That's just part of controlling variables to test for one thing. It also doesn't matter that they were fed processed food rather than having to find things in the wild, unless the effect of diet was the specific thing they were examining.

It does not matter how they give Alzheimer's to a lab mouse providing they give Alzheimer's to a lab mouse – not a different disorder that happens to have the symptoms of Alzheimer's.